The commonly assigned French patent FR 2,558,263 discloses the fabrication of micromechanical devices by microelectronic techniques. These methods produce a microstructure, which is defined as a mechanical structure produced using microelectronic techniques, that is to say collective processing of substrates by deposition, etching, photolithography, etc.
In the case of accelerometers, the electrically active portion can be partly carried by a mobile member, and in this case acceleration is measured by measuring a variation in the electrical capacitance between the mobile member and a fixed portion of the mechanical microstructure, for example.
The above kind of microstructure can include a cap to confine the active portion, as disclosed in particular in the commonly assigned French patent FR 2,770,339.
During this development process, the inventors were first confronted with the problem of stray capacitances appearing between the active portion and the cap. It is necessary to reduce the stray capacitances, especially if a highly sensitive sensor is required.
To this end, and in accordance with a first feature of the invention disclosed in FR 2,770,339, the inventors formed a recess in the cap, vertically aligned with and at a predetermined distance from the active portion.
The predetermined distance is in particular sufficient to limit or even prevent the appearance of the stray capacitance previously cited.
Furthermore, and as explained in the French patent FR 2,770,339, the electrically active portion can be produced in a layer formed on a substrate including one or more holes opening onto an internal face of the layer, through which an electrical contact is made with the electrically active portion on the internal face of the layer.
The electrical contact can be made with a metal deposit, for example using an ultrasound welding technique known to the person skilled in the art.
The inventors of the subject matter disclosed in FR 2,770,339 were confronted with problems related to the small thickness of the layer in which the active portion is produced (for example 60 μm); and more particularly to its great fragility.
During their research, they found that deterioration could occur, especially at the moment of producing the electrical contact with the internal face of the layer, as, if the external face of the layer is facing the cavity, the ultrasound vibration combined with the pressure of the welding device is likely to lead to rupturing of the thin layer, because it is very fragile.
They further found that providing too great a predetermined distance, in accordance with the first feature of the invention referred to above, was likely to encourage such rupturing, as it is difficult if not impossible to arrive at a compromise between minimizing the stray capacitances and avoiding rupture.